This invention relates generally to gas turbine engines, and more specifically to methods and systems for ordering blades on a rotor of a rotatable machines.
Gas turbines are used in different operating environments, such as, to provide propulsion for aircraft and/or to produce power in both land-based and sea-borne power systems. At least some known gas turbine engines include a core engine having, in serial flow arrangement, a fan assembly and a high pressure compressor that compress airflow entering the engine. A combustor ignites a fuel-air mixture that is then channeled through a turbine nozzle assembly towards high and low pressure turbines. The turbines each include a plurality of rotor blades that extract rotational energy from airflow exiting the combustor.
At least some known turbofan gas turbine engines include a fan assembly that includes a plurality of fan blades extending radially outwardly therefrom. These blades may operate with a relative blade tip Mach number in the transonic regime and may be subject to an operating characteristic called multiple-pure-tone (MPT) noise, sometimes referred to as buzzsaw noise. Buzzsaw noise may occur if at least some blades are oriented differently relative to other blades extending around the circumference of the fan case. Moreover, such noise may occur if blade-to-blade geometry variations exist within the fan and/or if flowfield disturbances are present forward of the fan inlet. Such flowfield disturbances may be caused by any number of factors including, but not limited to drain leakage, panel splice leakage, or other geometric nonuniformities. As a result, variations may exist within the fan assembly in the amplitude (strength) and/or spacing of the shockwaves originating from those portions of the blades that have sonic or supersonic velocities. Specifically, at axial locations close to the fan blades, the noise due to the shock waves is generally at multiples of the fan shaft per revolution frequency, which is the frequency with which one point on the shaft passes any particular fixed point as it rotates.
Shock waves of different strengths based on the flowfield disturbances may propagate at different speeds. Accordingly, as the shock waves travel away from the blades, the noise at a blade passing frequency degenerates into a broad spectrum of lower frequency tones as the shock waves merge with each other. Such tones or buzzsaw noise tend to increase passenger annoyance and reduce passenger comfort, and may also adversely affect community noise levels. However, maintaining close manufacturing tolerances to minimize blade to blade orientation and/or geometry variations may be insufficiently capable of minimizing buzzsaw noise.